Support desktop tasks in recents animation

Desktop tasks are using freeform windowing mode. Update recents
animation to support freeform tasks when desktop mode feature flag is
enabled.

Changes:
- set initial size for freeform tasks to be the size of the thumbnail
  instead of size of the screen
- support multiple individual remote animation targets when starting the
  recents animation

TODO:
- there are flickers when starting and ending the recents animation

Bug: 263264985
Test: swipe up when more than 1 desktop task is visible
Change-Id: I27ee02774281b3a433d779c0bb8825cdb6ea5457